A conventionally known torque converter includes a lockup clutch, a torsional vibration damper, and a rotary inertia mass damper (power transmission mechanism) with a planetary gear (as shown in, for example, Patent Literature 1). In the torsional vibration damper of the torque converter has two cover plates (input element) are respectively coupled with a lockup piston by means of a plurality of bearing journals, a sun gear disposed between the two cover plates in an axial direction thereof such as to work as a driven-side transmission element (output element), and springs (elastic bodies) which transmit a torque between the cover plates and the sun gear. In addition to the sun gear, the rotary inertia mass damper further has a plurality of pinion gears (planet gears) rotatably supported by the cover plates as carrier via the bearing journals such as to mesh with the sun gear, and a ring gear that meshes with the plurality of pinion gears. In the above conventional torque converter, when the lockup clutch is engaged and the cover plates of the torsional vibration damper is rotated (twisted) relative to the sun gear, the springs are deflected and the ring gear as the mass body is rotated in accordance with relative rotation of the cover plates and the sun gear. This configuration causes an inertia torque according to a difference in angular acceleration between the cover plates and the sun gear to be applied to the sun gear as the output element of the torsional vibration damper from the ring gear as the mass body via pinion gears and improves the vibration damping performance of the torsional vibration damper.